Convection Velocity of Vortex Structures in the Near Wake of a Circular Cylinder

The convection velocity of vortex structures in the near wake of a circular cylinder was experimentally investigated over the region $1.6-2.5⩽x/D⩽12.0$ for $R=160-12\text{,}000\text{.}$ Dye injection technique of flow visualization and two completely noninvasive laser Doppler velocimeters were employed for $R⩽320$ and ⩾400, respectively. The convection velocity, $U_c\text{,}$ is defined as the mean traveling velocity of vortex cores passing a streamwise separation during a mean elapsed time. For $R⩽320\text{,}$ $U_c$ was determined directly from the motion of dye-marked vortex cores filmed by a video camera. In the cases of $R⩾400\text{,}$ the positions of peak vorticity and half of the half-velocity-defect width at each downstream section were first used to identify the mean path of vortex cores (i.e., the most probable trajectory of the vortex structures), along which spatial correlation measurements were then performed to determine the mean elapsed time corresponding to the maximum cross correlation. The present results show that, in laminar and transitional wakes, the ratio $U_c/U_o$ increases from 0.53 to 0.84 over a region of $1.6⩽x/D⩽6.0$ and then tends to be a constant of 0.84 for $x/D⩾6.0\text{.}$ In a turbulent wake, $U_c/U_o$ also increases from a certain value at a point downstream from the position of vortex formation to a mean value of about 0.86 at $x/D⩾5.0-6.0\text{,}$ and then changes little with the increase of $x/D\text{.}$ In addition, it is found that the dependence of $U_c/U_o$ on $R$ almost disappears for $x/D⩾5.0\text{.}$